Introduction

The Sigma AF 20mm f/1.8 EX owns one of the industries records - with its
max. aperture of f/1.8 it is the very fastest 20mm mass production lens
around! As of
today the lens is available for the classic systems as well as in native
Sigma mount. Sigma has recently released a DG version of the lens featuring
extra coating on the rear elements for better flare performance but other
than that the principal design remained unaltered so most of the findings
should also be valid for this new variant.
In the scope of this report the lens was tested using an APS-C DSLR
resulting in a full frame equivalent of 32mm.

Thanks to the ultra-large aperture and the corresponding need for large diameter
glass elements the lens is rather massive with a weight of 520g and a size of
89x90mm. The filter size is a whopping 82mm and it is a good idea to use
one to protect the super-sized front element. As illustrated in the product
images above the package also includes a dedicated petal-type hood.
The optical construction is made of 13 elements in 11 groups with
two aspherical elements and 9 aperture blades.
The min. focus distance is 0.20m resulting in a surprisingly large max. object
magnification of 1:4.

The lens incorporates a Dual-Focus (DF) mechanism which allows
you to decouple the focus ring from the focusing gear by pulling/pushing it
back and forth. The idea behind this remains somewhat mysterious to me because you
still have to switch the lens from AF to manual focusing - independent from the dual-focus
mechanism that is. So with two switches (DF & AF/MF) you've four different behaviors:

DF SWITCH POSITION

AF SWITCH

RESULT

FRONT

AF

AF with rotating focus ring

BACK

AF

AF with non-rotating focus ring

FRONT

MF

manual focusing via focus ring

BACK

MF

neither manual nor auto-focusing

To me it seems as if Sigma is trying to market a mechanical workaround as a feature.
Most better quality lenses don't have a rotating focus ring in AF mode anyway so
what's the advantage of DF compared to convention focusing mechanism ? Tokina's
one-touch focus-clutch mechanism used in some of their AT-X Pro lenses is
similar but it combines DF with a change of the focusing mode which makes more
sense to me.

Apart from DF the construction quality is decent using the typical
EX ("Excellence") finish (crinkle style). The very broad, rubberized
focus ring has a smooth action but unfortunately it feels a little wobbly
when decoupled using the DF mechanism.